A 75 Se-labeled protein that absorbs strongly in the UV range (UV max equals 257 nm) is formed by C. sticklandii and C. sporogenes in the presence of chloramphenicol and rifampicin. Under the same conditions the biosynthesis of the selenoprotein A component of glycine reductase is inhibited. The 257 nm Se-protein is cleaved into smaller fragments by proteases and by ribonuclease. The possible relationship of this unidentified Se-protein to selenoprotein A is under investigation. The larger molecular weight glycine reductase proteins, protein B and fraction C were further characterized and reconstitution of the complex was studied.